1. Field of the Invention
The present invention relates to an arrayed I/O unit close decision method and apparatus and a medium containing an arrayed I/O unit close decision program and in particular, to an arrayed I/O unit close decision method and apparatus for controlling a timing to close out from a system an arrayed I/O unit which has frequently caused an operation failure, as well as a medium containing an arrayed I/O unit close decision program.
2. Description of the Related Art
Suppose as an arrayed I/O apparatus, a disc array system in which a plurality of disc units are arranged in an array. Each of the arrayed disc units has normally a retry circuit. When one of the arrayed I/O units is accessed and a data read-out or write-in fails, the retry circuit tries to repeat the read-out or write-in operation. If this retrial is successful, i.e., the read-out or the write-in operation is completed successfully by this retrial, the currently executed processing is continued as it is.
On the other hand, if the retrial is unsuccessful and a predetermined number of retrials is reached, it is decided that the operation failure cannot be eliminated by retrial and the arrayed disc unit accessed is closed out from the system.
Here, the retry circuit executing the retrial should execute various processes required for the retrial operation such as an initial error detection, an error latch release, a fall back operation for re-executing a transaction, a mechanical positioning, and the like. For this, the processing time required for retrial is usually 100 times more than a processing time required for a normal access operation.
However, in the aforementioned conventional example, even if the number of retrials is limited to once, the disc unit which has caused an access error will not be closed from the system if the access error is eliminated by one retrial. Accordingly, if that disc unit is frequently accessed, the retrial is repeated each time, increasing the average transaction processing time, disabling the practical use of the system.
Suppose one retrial requires a time equal to N times of a time Tn required for a normal operation. If an operation failure occurs with a probability P, an average transaction processing time Tv is calculated as follows. ##EQU1##
Here, the second term in Expression (1) represents a transaction processing delay time caused by an operation failure. If the value of N becomes 100 times, the average transaction processing time is doubled even if the P is 1%. Suppose P is 50% which is a conventional limit value for close, then the average transaction processing time becomes 50 times more than a normal operation. That is, the aforementioned system cannot be used in practice.